While being applicable to arbitrary fibre compound structures the present invention as well as the underlying problem is explained in more detail with regard to carbon fibre reinforced plastic components of an aeroplane structure.
For weight reduction reasons the components of aeroplanes are increasingly made of fibre compound materials, particularly made of carbon fibre reinforced plastics (CFRP). Thereby they comprise a structure of multiple layers of fibre lay-up which are connected to each other by means of a hardened resin matrix, particularly an epoxy resin matrix. For producing such components according to conventional methods multiple layers of impregnated fibre lay-ups are placed in a laminating device. Subsequently the resin matrix is hardened, for instance by exposure to elevated temperature and elevated pressure.
According to a conventional method a component is enclosed in a bag of pressure-tight foil whose interior is evacuated by means of a vacuum pump. Such enclosed component is then inserted into an autoclave where it is exposed to elevated pressure.
The published patent application US2007/0080481 describes a method for producing fibre compound components in which an expansion element together with compound material is inserted into a closable container. The container is closed and the expansion element is exposed to overpressure which presses the compound material against an outer wall of the container.
In case of rectification work on such fibre compound components, e.g. for correcting defects, for repairing damages or for modifying the components, in a conventional working process preimpregnated fibre layers (prepregs) are fixed by means of an adhesive tape on the surface to be processed of the component and are covered with a pressure-tight membrane as for instance a vacuum bag. The area enclosed under the membrane is evacuated and the component is inserted in to an autoclave where it is exposed to elevated pressure for hardening.
Here the problem arises that also for small, locally bounded rectification works of a component an autoclave has to be provided which is big enough to accommodate the whole component. This results in considerable costs, particularly for relatively big components as for instance fuselage shells or stiffening elements of fuselages, which multiply if erroneous rectification works have to be repeated. If present, sufficiently big dimensioned autoclaves are occupied for other purposes time gets lost due to waiting. Another drawback is that in most cases the components have to be reinserted into the device in which they were manufactured during the hardening in the autoclave. During the rectification work the device is not available for the production run. Also, in case of rectification works on mounted assemblies these cannot be inserted into the autoclave if they are too big or if they contain parts as for instance electric installations which would be damaged by high pressures.
If the fibre layers are hardened without application of an autoclave and if thereby merely the area enclosed under the membrane is evacuated the fibre layers are exposed to a maximal pressure difference between inner pressure and outer pressure of 0.8-0.9 bar. Under these pressure conditions adhesive films tend to pore generation which worsens the stability of the components and leads to error displays during an ultrasonic inspection which is required for a quality check.
DE 40 19 744 A1 discloses rubber stamps for applying pressure during repair work of fibre compound structures. However, due to the locally uneven pressure distribution an application of such mechanical clamping elements regularly leads to an uneven thickness of the hardened fibre layers and therefore to an insufficient precision of rectification works. Further, DE 40 19 744 A1 also discloses to place a bell-shaped top over a membrane that covers a repair location, and then to exert overpressure within the bell-shaped top upon the repair location. However, since the overpressure leads to forces that tend to drive the bell-shaped top and the membrane away from each other, it is difficult to reliably seal the contact line between the bell-shaped top and the membrane. Furthermore, the method cannot be applied in cases where covering the repair location with a foil or membrane is not desired.